A sidelink communication mode refers to that a packet needs not to pass a core network and a base station, and a communication link may be directly established between UE1 and UE2 for communication. And a discovery process is generally performed before sidelink communication is performed. For example, UE2 needs first to discover whether UE1 is near before transmitting information to UE1 in the sidelink communication mode. FIG. 1 is a schematic diagram of discovery or sidelink communication establishment by two UEs which are both under coverage of an eNB. FIG. 2 is a schematic diagram of discovery or sidelink communication establishment by one UE under coverage of an eNB and another UE not under the coverage of the eNB. And FIG. 3 is a schematic diagram of discovery or sidelink communication establishment by two UEs which are not under coverage of an eNB.
The sidelink communication mode may be used to expand coverage of a cell. As shown in FIG. 1, UE2 is located at an edge of a cell, and the signals received by it are relatively weak, thus it is very possible to that the UE2 will leave the coverage of the cell. Furthermore, there exists a terminal nearby which is authorized by a network and has a function of relay, referred to as a relay, such as UE1. And in order to avoid traffic interruption, UE2 may access to a base station via the relay UE1 after discovering the relay UE1, and proceed with performing normal traffic communication. As shown in FIG. 2, UE2 is located in an area with no network coverage, and there exists a terminal nearby which is authorized by a network and has a function of relay, such as UE1. And in order to access to a network for communication, UE2 may access to a base station via the relay UE1 in coverage of the cell after discovering the relay UE1, and proceed with performing normal traffic communication.
It should be noted that the above description of the background is merely provided for clear and complete explanation of this disclosure and for easy understanding by those skilled in the art. And it should not be understood that the above technical solution is known to those skilled in the art as it is described in the background of this disclosure.